As known, a bicycle bottom bracket assembly comprises a shaft rotatably supported in a housing box provided in the bicycle frame and two crank arms associated with the opposite ends of the shaft. The shaft can be made in a distinct piece from the crank arms or can be made in a single piece with one of the two crank arms.
The rotation of the shaft with respect to the box is achieved through the use of a pair of rolling bearings mounted on the shaft. Specifically, each bearing is positioned on the shaft at a respective body portion of the shaft adjacent to the crank arm.
Typically, the bearings are supported inside or outside the box provided in the bicycle frame by suitable adapters mounted at the opposite free ends of such a box.
It is known that the precision of gearshifting is greatly influenced by the distance between the middle plane of the frame and the middle plane of the crankset, also known as the chain line.
In the known assembly great attention is put on the chain line at the design stage and assembly stage.
However, the Applicant has found that often, in currently known assemblies, the shifting of the gear engaged with the chain is not very precise.